P20B9 on Chevrolet
Reductant Heater "A" Control Circuit/Open
P20B9 on Chevrolet vehicles indicates reductant heater "a" control circuit/open. Au7o has documented this code across 1 Chevrolet model — most commonly on Silverado 3500HD. This code applies to diesel vehicles with Selective Catalytic Reduction (SCR) and means the engine computer detected an open circuit in the control for reductant (Diesel Exhaust Fluid / DEF) heater "A". This heater keeps the DEF and its lines from freezing or crystallizing in cold weather so the emissions system can function. An open control circuit means the heater can't be commanded properly, which can cause DEF freezing, SCR faults, and eventually reduced power or limited engine starts to enforce emissions compliance. It should be addressed promptly, especially in cold climates. Typical repair costs on Chevrolet range from $300 to $2,500, depending on the specific model and root cause.
Common Causes of P20B9
- •Open or broken wiring in the reductant heater circuit
- •Corroded or loose heater connector
- •Failed reductant (DEF) tank heater element
- •Blown fuse or relay for the heater circuit
- •Faulty DEF heater control module
- •PCM driver circuit fault (rare)
P20B9 on Chevrolet by Model
Chevrolet Silverado 3500HD(1 issue)
- DEF System and SCR Catalyst Failures2011-2025
The Diesel Exhaust Fluid (DEF) system is plagued with issues including DEF tank heater failure, injector clogging, quality sensor malfunctions, and SCR catalyst degradation. Any failure triggers a countdown timer that eventually limits the truck to 5 mph if not addressed. The system is sensitive to DEF quality and crystallization.
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View P20B9 across all makes →Frequently Asked Questions
What does P20B9 mean on Chevrolet?▼
P20B9 stands for "Reductant Heater "A" Control Circuit/Open." This code applies to diesel vehicles with Selective Catalytic Reduction (SCR) and means the engine computer detected an open circuit in the control for reductant (Diesel Exhaust Fluid / DEF) heater "A". This heater keeps the DEF and its lines from freezing or crystallizing in cold weather so the emissions system can function. An open control circuit means the heater can't be commanded properly, which can cause DEF freezing, SCR faults, and eventually reduced power or limited engine starts to enforce emissions compliance. It should be addressed promptly, especially in cold climates. On Chevrolet specifically, this code is documented across 1 model.
What causes P20B9 on Chevrolet vehicles?▼
Common causes on Chevrolet: Open or broken wiring in the reductant heater circuit, Corroded or loose heater connector, Failed reductant (DEF) tank heater element, Blown fuse or relay for the heater circuit, Faulty DEF heater control module. Specific causes vary by model and year — see the per-model sections below.
How much does it cost to fix P20B9 on a Chevrolet?▼
Repair costs on Chevrolet range from $300 to $2,500, depending on the specific model and root cause.
Which Chevrolet models have P20B9 documented?▼
Au7o has documented P20B9 on 1 Chevrolet model: Silverado 3500HD.